1.1.2Development of Practical Skills in Physics

Control Variables

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Lesson summary

1.1.2 Control Variables — OCR A Level Physics A notes: Identify the variables that must be controlled in an experiment and describe practical ways of keeping them constant.A fair test changes only one quantity at a time.

Temperature
Amplitude or starting position
Material and dimensions
Voltage or current
Environment
control experiment

Section 1 of 9Why control variables matter

A fair test changes only one quantity at a time. If two quantities change together, a change in the result cannot be assigned to either, and the experiment is not valid. Variables that you do not want to influence the result are called control variables, and a good plan lists them and explains how each is kept constant.

The first step is to ask, from the physics, which quantities appear in the relevant equation or could physically affect the outcome. For the resistance of a wire, R = ρL/A: if you vary L, you must keep the material, the cross-sectional area and the temperature fixed.

Section 2 of 9Common control variables and how to control them

1Temperature

Temperature use a low current and switch off between readings so a wire does not heat up; for thermal experiments use a water bath and stirrer, or a lagged container.

2Mass

Mass use the same trolley or add equal masses to each side; check with a balance.

3Amplitude or starting position

Amplitude or starting position release from the same mark each time, or use a fixed release height.

4Material and dimensions

Material and dimensions use samples cut from the same reel; measure diameters with a micrometer at several places.

5Voltage or current

Voltage or current use a stabilised supply or a variable resistor with an ammeter to hold the current at a set value.

6Environment

Environment ambient light level, draughts, background radiation (measured and subtracted).

Section 3 of 9Variables that cannot be fully controlled

Some factors cannot be held perfectly constant.

Variables that cannot be fully controlled

Room temperature drifts, a battery slowly runs down, and a human reaction time changes.

The plan should say how the effect is minimised or monitored: record the temperature at the start and end; use a mains power supply instead of a battery; use light gates instead of a stopwatch.

Where a variable cannot be controlled, it should be measured so that its possible influence can be commented on in the evaluation.

A control variable is sometimes confused with a control experiment. A control experiment is a separate run in which the independent variable is not applied, giving a baseline for comparison. An example is measuring the count rate with no source present, which is the background count rate.

Section 4 of 9Controlling several variables in one investigation

Many A Level experiments investigate two relationships in turn. To study how the terminal velocity of a ball bearing in oil depends on radius and on temperature, you would run one series varying radius at constant temperature, and a second varying temperature with the same ball. For each series, the other variable is a control variable.

If the plan involves a graph, linking the control variables to the gradient is useful: the gradient is only meaningful if everything it depends on has been held constant during the series.

Section 5 of 9Linking to uncertainty

Imperfect control adds to scatter in the data. If repeated readings at the same setting differ more than the instrument resolution suggests, an uncontrolled variable is often responsible. This is a clue to look for in the evaluation of an experiment (see 1.2.4 and 1.2.9).

Section 6 of 9Spotting the hidden variable

A skill that is often tested is spotting a variable that the student has not realised is changing.

In an investigation of the cooling of hot water, using a larger volume in one beaker changes the surface area and the mass at the same time; using a different thermometer depth changes the temperature reading.

In a light experiment, the brightness of the room, the distance from the source and the angle of the detector may all influence a light-dependent resistor.

A useful habit is to write the equation that links the quantities and check every symbol on the right-hand side.

Any symbol that is not the independent variable must either be constant or be measured.

If the equation is not known, ask what physical processes could change the result and treat each as a potential control variable.

Finally, remember that a controlled variable should be controlled for the whole experiment, not just at the beginning.

Section 7 of 9Worked examples

A student investigates how the time t for a metal ball to fall a fixed distance through a viscous liquid depends on the ball's diameter d. State four control variables and how each is controlled.
In an experiment on the resistance of a wire, the student leaves the current switched on throughout and finds the resistance for the longer lengths is larger than expected. Suggest why and what to do.

Section 8 of 9Key terms

Term	Meaning
Control variable	A variable kept constant so only the independent variable affects the result.
Control experiment	A run without the independent variable applied, giving a baseline.
Fair test	One in which only the independent variable changes.
Background count	Radiation detected with no source present, to be subtracted.
Lagging	Insulation used to reduce thermal energy transfer.

Section 9 of 9Exam technique

Listing control variables but not explaining how they are kept constant.

Confusing a control variable with the dependent variable or with a control experiment.

Forgetting temperature as a control variable in electrical and thermal experiments.

Assuming a variable is controlled because it was constant in theory, without checking it with an instrument.

Changing two variables at once to save time.